I Got a Cheat Skill in Another World and Became Unrivaled in the Real World, Too (Manga), Vol. 1 by Miku

Yuuya has always been a loser, and after the death of his grandpa (the only person he had in his corner), he’s sure there’s no point even trying anymore. But when Yuuya moves into his late gramps’s home, he inherits...a two-way door to another world?! Access to this new realm might be just the cheat Yuuya needs to succeed in his everyday life!